Temperature evolution inside a pot during experimental surface (bonfire) firing

Time–temperature evolutions of different parts inside a pot were recorded during three experimental surface (bonfire) firings. The experimental vessels were shaped from a calcareous clay, tempered with 30% vol. of oolithic limestone. The temperature–time recording showed: (1) Tmax. was reached after 12–22 min and differed between the firings as well as inside individual vessels; (2) the range of the thermal variation within one single firing was found to be as high as 390 °C, and up to 220 °C on a specific cross-section; (3) the lowest temperature was not systematically recorded in the core of the object, as is generally expected. Under the polarizing microscope, no textural and mineralogical changes were observed in the ceramic bodies. This is sustained by powder X-ray diffraction analyses evidencing no dolomite or calcite breakdown. The presence or absence of specific illite and chlorite peaks can be generally related to Tmax. and soaking time, but equivalent firing temperature estimations do not match the measured temperatures.

Research Highlights
► Maximum temperatures of 800–900 °C were reached after 12–22 min.
► Range of thermal variation within one single firing is as high as 390 °C.
► No textural and mineralogical changes were observed with OM and XRD.
► Equivalent firing temperature estimations do not match the measured temperatures.